Exhibit 5.1

Nicole C. Brookshire

T:+1 212 479 6157

nbrookshire@cooley.com

March 18, 2022

MongoDB, Inc.

1633 Broadway, 38th Floor

New York, NY 10019

Re: Registration on Form S-8

Ladies and Gentlemen:

We have acted as counsel to MongoDB, Inc., a Delaware corporation (the “Company”), in connection with the filing of a Registration Statement on Form S-8 (the “Registration Statement”) with the U.S. Securities and Exchange Commission, covering the offering of up to 4,046,662 shares (the “Shares”) of the Company’s Class A common stock, par value $0.001 per share (the “Class A Common Stock”), consisting of (a) 3,372,218 shares of Class A Common Stock issuable pursuant to the Company’s 2016 Equity Incentive Plan and (b) 674,444 shares of Class A Common Stock issuable pursuant to the Company’s 2017 Employee Stock Purchase Plan (together with the Company’s 2016 Equity Incentive Plan, the “Plans”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and related prospectuses, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, (c) the Plans and (d) originals or copies certified to our satisfaction of such other records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies, the accuracy, completeness and authenticity of certificates of public officials; and the due authorization, execution and delivery of all documents by all persons other than the Company where authorization, execution and delivery are prerequisites to the effectiveness of such documents. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not independently verified such matters.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion to the extent that any other laws are applicable to the subject matter hereof and express no opinion and provide no assurance as to compliance with any federal or state securities law, rule or regulation.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued in accordance with the Plans, the Registration Statement and related prospectuses, will be validly issued, fully paid, and nonassessable (except as to Shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

Cooley LLP 500 Boylston Street 14th Floor Boston, MA 02116-3736

t: +1 617 937 2300 f: +1 617 937 2400 cooley.com

MongoDB, Inc.

March 18, 2022

Page Two

We consent to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

Cooley LLP

By: /s/ Nicole C. Brookshire

Nicole C. Brookshire

Cooley LLP 500 Boylston Street 14th Floor Boston, MA 02116-3736

t: +1 617 937 2300 f: +1 617 937 2400 cooley.com